This invention relates to the field of nonwoven fabrics. The manufacture of nonwoven fabrics like meltblown and spunbond fabrics involves the attenuation of polymer streams, generally in a fluid such as air. In spunbond fiber production, for example, fibers are attenuated within a chamber called a drawing unit and deposited onto a moving conveyor belt called a forming wire. In meltblown fiber production fibers the drawing unit usually consists of only a nozzle through which polymer flows and is then attenuated pneumatically before deposition onto the forming wire.
One of the characteristics of certain types of nonwoven fabrics is the uniformity of formation. Non-uniformity can result in varying properties in a given length of nonwoven fabric and cause premature failure of the fabric and/or unsatisfactory appearance of tactile properties. Increasing uniformity should increase the force a nonwoven fabric may withstand prior to failure, i.e. the fabric should be stronger. Fabrics which are, pound for pound, stronger than other fabrics, will allow the products into which they are made to be thinner and lighter weight at the same strength level or simply stronger at the same basis weight.
Though the inventors do not wish to be bound by this belief, some of them believe that one of the impediments to producing a stronger nonwoven fabric is the large scale turbulence produced in the drawing chamber by the large amount of air moving through it along with the fibers. They believe that large scale turbulence disrupts the smooth flow of fibers from the spinneret to the forming wire and so introduces non-uniformities and other areas of weakness within the web. Alternatively, some of the inventors believe that not decreasing but increasing the turbulence in the drawing chamber will result in more shifting of the fibers and so therefore more uniform web production. The exact theory of operation remains undefined, however, the webs produced according to this invention are more uniform than similar webs produced without the use of this invention.
Accordingly, it is an object of this invention to provide a nonwoven fabric which is produced in a novel way which increases web uniformity. The increase in uniformity increases the strength of the nonwoven web.